Thermal spray
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Thermal Spray techniques are coating processes in which melted (or heated) materials are sprayed onto a surface. The "feedstock" (coating precursor) is heated by electrical (plasma or arc) or chemical means (combustion flame). Coating thicknesses range between approximately 20 micrometers (μm) and several millimetres (mm) depending on the process and feedstock. Thermal spraying is a line-of-sight process and typically can not be used to coat holes and deep recesses on a work piece.
The materials to be deposited as the coating are typically fed into the spray gun in powder, wire or rod form where they may be atomized before being accelerated towards the substrate, or material to be coated[1]. "As the sprayed particles impinge upon the surface, they cool and build up, splat by splat, into a laminar structure forming the thermal spray coating."[2] The surface may not heat up significantly, allowing the coating of flammable substances.
Coating quality is usually assessed by measuring its porosity, oxide content, macro and microhardness, bond strength and surface roughness. Generally, the coating quality increases with increasing particle velocities.
Spray coatings may either be applied manually or by machine, depending on complexity, cost, and environmental[3] and safety concerns.
Contents |
[edit] Applications[4]
- Corrosion protection
- Fouling protection
- Altering conductivity
- Hardfacing
- Wear control
- Repairing damaged surfaces
- Temperature/oxidation protection (thermal barrier coatings)
- Medical implants
[edit] Typical methods
- Flame spraying (with powder and wires)
- High Velocity Oxygen-Fuel (HVOF)
- Detonation gun (D-Gun)
- Arc spraying
- Plasma spraying with powders under atmospheric and vacuum or low pressure conditions
- DC-arc plasma spray
- RF induction plasma spray
[edit] Safety
Thermal spraying need not be a dangerous process, if the equipment is treated with care, and correct spraying practices are followed. As with any industrial process, there are a number of hazards, of which the operator should be aware, and against which specific precautions should be taken.
Ideally, equipment should be operated automatically, in enclosures specially designed to extract fumes, reduce noise levels, and present direct viewing of the spraying head. Such techniques will also produce coatings that are more consistent. There are occasions when the type of components being treated, or their low production levels, require manual equipment operation. Under these conditions, a number of hazards, peculiar to thermal spraying, are experienced, in addition to those commonly encountered in production or processing industries.
[edit] Noise
Metal spraying equipment uses compressed gases, which create noise. Sound levels vary with the type of spraying equipment, the material being sprayed, and the operating parameters. Typical sound pressure levels taken 1 meter behind the arc.
[edit] UV light
Combustion spraying equipment produces an intense flame, which may have a peak temperature more than 3,100°C, and is very bright. Electric arc spraying produces ultra-violet light, which may damage delicate body tissues. Spray booths, and enclosures, should be fitted with ultra-violet absorbent dark glass. Where this is not possible, operators, and others in the vicinity, should wear protective goggles containing BS grade 6 green glass. Opaque screens should be placed around spraying areas. The nozzle of an arc pistol should never be viewed directly, unless it is certain that no power is available to the equipment.
[edit] Dust and fumes
The atomization of molten materials, produces a certain amount of dust and fumes. Proper extraction facilities are vital, not only for personal safety, but to minimize entrapment of re-frozen particles in the sprayed coatings. The use of breathing masks, fitted with suitable filters, is strongly recommended, where equipment cannot be isolated. Certain materials offer specific known hazards.
1. Finely divided metal particles are potentially pyrophorric and none should be allowed to accumulate.
2. Certain materials e.g. aluminum, zinc and other base metals may react with water to evolve hydrogen. This is potentially explosive and special precautions are necessary in fume extraction equipment.
3. Fumes of certain materials, notably zinc and copper alloys are unpleasant to smell, and, in certain individuals, may cause a fever-type reaction. This may occur some time after spraying and usually subsides rapidly. If it does not, medical advice must be sought.
[edit] Heat
Combustion spraying guns use oxygen and fuel gases. The fuel gases are potentially explosive. In particular, acetylene may only be used under approved conditions. Oxygen, while not explosive, will sustain combustion, and many materials will spontaneously ignite, if excessive oxygen levels are present. Care must be taken to avoid leakage, and to isolate oxygen and fuel gas supplies, when not in use.
[edit] Shock hazards
Electric arc guns operate at low voltages (below 45 dc), but at relatively high currents. They may be safely hand-held. The power supply units are connected to 440 volt AC sources, and must be treated with caution.
[edit] References
- ^ Sulzer Metco website describing the processes and applications[1]
- ^ Army manual describing spray coating - http://www.usace.army.mil/usace-docs/eng-manuals/em1110-2-3401/c-2.pdf - Accessed 20 June 2006.
- ^ Article discusses briefly several environmental concerns and provides links at the bottom to industrial pollution papers - [2] - Accessed 20 June 2006.
- ^ Army manual
